discovery: fix mocked peer in unit tests

The mocked peer used here blocks on `sendToPeer`, which is not the
behavior of the `SendMessageLazy` of `lnpeer.Peer`. To reflect the
reality, we now make sure the `sendToPeer` is non-blocking in the tests.

discovery/syncer.go

@@ -495,6 +495,8 @@ func (g *GossipSyncer) handleSyncingChans() {
 	g.Lock()
 	defer g.Unlock()
 
+	// Send the msg to the remote peer, which is non-blocking as
+	// `sendToPeer` only queues the msg in Brontide.
 	err = g.cfg.sendToPeer(queryRangeMsg)
 	if err != nil {
 		log.Errorf("Unable to send chan range query: %v", err)